U.S. Pat. No. 4,302,357 relates to an activated alumina catalyst employed in a process for the production of ethylene from ethanol through a dehydration reaction. In the description LHSV of ethanol is from 0.25 to 5 h−1 and preferably from 0.5 to 3 h−1. The examples are carried out at 370° C. and LHSV of 1 h−1, ethylene yield is from 65 to 94%.
Process Economics Reviews PEP' 79-3 (SRI international) of December 1979 describes the dehydration of an ethanol-water (95/5 weight %) mixture on a silica-alumina catalyst in a tubular fixed bed at 315-360° C., 1.7 bar absolute and a WHSV (on ethanol) of 0.3 h−1. The ethanol conversion is 99% and the ethylene selectivity is 94.95%. It also describes the dehydration of an ethanol-water (95/5 weight %) mixture on a silica-alumina catalyst in a fluidized bed at 399° C., 1.7 bar absolute and a WHSV (on ethanol) of 0.7 h−1. The ethanol conversion is 99.6% and the ethylene selectivity is 99.3%.
U.S. Pat. No. 4,873,392 describes a process for converting diluted ethanol to ethylene which comprises heating an ethanol-containing fermentation broth thereby to vaporize a mixture of ethanol and water and contacting said vaporized mixture with a ZSM-5 zeolite catalyst selected from the group consisting of:                a ZSM-5 zeolite having a Si/Al atomic ratio of from 5 to 75 which has been treated with steam at a temperature ranging from 400 to 800° C. for a period of from 1 to 48 hours;        a ZSM-5 zeolite having a Si/Al atomic ratio of from 5 to 50 and wherein La or Ce ions have been incorporated in a weight percentage of 0.1 to 1.0% by ion exchange or in a weight percentage ranging from 0.1 to 5% by impregnation, and        a ZSM-5 zeolite having a Si/Al of from 5 to 50 and impregnated with a 0.5 to 7 wt % of trifluoromethanesulfonic acid,and recovering the ethylene thus produced.        
In ex 1 the catalyst is a steamed ZSM-5 having a Si/Al ratio of 21, the aqueous feed contains 10 w % of ethanol and 2 w % of glucose, the temperature is 275° C., the WHSV is from 3.2 to 38.5 h−1. The ethylene yield decreases with the increase of WHSV. The ethylene yield is 99.4% when WHSV is 3.2 h−1 and 20.1% when WHSV is 38.5 h−1.
In ex 2 a ZSM-5 having a Si/Al ratio of 10 is compared with the same but on which La or Ce ions have been incorporated. The aqueous feed contains 10 w % of ethanol and 2 w % of glucose, the temperature is from 200° C. to 225° C., the WHSV is 1 h−1 and the best ethylene yield is 94.9%.
In ex 3 the catalyst is a ZSM-5 having a Si/Al ratio of 10 on which trifluoromethanesulfonic acid has been incorporated, the aqueous feed contains 10 w % of ethanol and 2 w % of glucose, the temperature is from 180° C. to 205° C., the WHSV is 1 h−1. The ethylene yield increases with temperature (73.3% at 180° C., 97.2% at 200° C.) and then decreases (95.8% at 205° C.).
In the ethanol dehydration processes, ethanol conversion is nearly complete. The increase of C2− selectivity while keeping high ethanol conversion is of importance to gain in process efficiency and to save expensive steps of downstream separation/purification. A convenient solution has been discovered to adjust the activity and selectivity of the catalyst by poisoning the unselective acid sites while keeping active the selective acidic sites. This can be achieved by an appropriate spiking of the alcohol feed with a neutralising agent. A particular characteristic of the present invention is that the amount of neutralising agent to maximise selectivity can be adjusted continuously and eventually completely omitted from the reaction section. Such event can occur when (i) the residence time of the feed in the catalytic reactor changes, (ii) when the feed composition changes and contains similar neutralising components or (iii) when the catalyst deactivates due to poising or coke lay down on the catalyst surface.
The moderation of the catalytic activity by feed spiking the feedstock is documented for other processes but not for alcohol dehydration.
For instance, U.S. Pat. No. 4,517,395 discloses the addition of fixed amounts of carbon monoxide (CO), which increases the selectivity of the hydrogenation process towards the conversion of conjugated and/or cumulative double bonds and for acetylenic triple bonds into monoene-containing mixtures of hydrocarbons, so as to avoid to a maximum extend any losses of monoenes by the formation of saturated hydrocarbons.
Another example is to find in U.S. Pat. No. 7,399,402 which describes the introduction of an ammonia precursor when hydrotreating a C4-C8 hydrocarbon feed rich in olefins and aromatics on a catalyst consisting of transition metals supported on refractory oxides. The introduction of the ammonia precursor into the feed allows to block the acid sites responsible for secondary reactions (oligomerization and alkylation on acid sites in this prior art), thus keeping excellent product quality.
In order to avoid the double bond isomerisation of the primary alpha-olefins in the dehydration of long-chain alcohols, the use of metal cations to modify the catalyst (by minimizing the number of acid sites that are thought to increase the rate of isomerization) have been reported (K. Jira'tova', L. Bera'nek, Appl. Catal. 2 (1982) 125; R. Miranda, D. J. Collins, J. Catal. 88 (1984) 542 and U.S. Pat. No. 4,234,752). Such methods are permanent, irreversible and hence no means are left available to adjust the performance when feed composition, feed residence time and catalyst activity changes over the time of using the catalyst.
U.S. Pat. No. 4,873,392 mentions at col 1 line 48-col 2 line 9 a modification of the ZSM-5 acid sites if the production of ethylene is desired. This part of U.S. Pat. No. 4,873,392 relates to the MTO reaction in which methanol is converted to a mixture of ethylene, propylene and higher hydrocarbons. It has nothing to see with the present invention which relates to the dehydration of alcohols on acidic catalysts to make the corresponding olefin which means an olefin having the same number of carbons as the alcohol precursor.